One important potential use of volume holograms is in digital data storage. The three dimensional nature of a volume hologram, which refers to the storage of each bit as a hologram extending throughout the entire volume of the recording medium, renders volume holograms suitable for use in high capacity digital data storage. A group of bits can be encoded and decoded together as a two dimensional array of bits referred to as a page.
In Holographic Data Storage (HDS) systems the optics configuration used for recording and reading typically comprises a Fourier Transform (FT) geometry that uses a 4f optical imaging system which for recording includes a spatial light modulator (SLM) or other optical encoding device that displays information.
Mutually coherent object and reference beams form an interference pattern in the volume of their overlap. A hologram is recorded when light-induced changes in the volume of their overlap in the storage medium, such as photopolymerization, produce a record of the resulting interference pattern. The essential elements and arrangement of the 4f optical design for recording holograms are: an SLM that encodes the object beam, a lens element L1, having an effective focal distance f1 and which is located at a distance f1 from the SLM in the optical path of the encoded object beam, for relaying a 2-D Fourier transform of the spatial pattern of the encoded object beam to a plane that is one focal distance from the lens and a holographic recording medium (HRM), located at a distance f1 from the lens element L1 in the optical path of the encoded object beam relayed by the lens element L1.
Reconstruction of a hologram is accomplished by firstly illuminating the hologram with a reference beam that is substantially the same as the reference beam used for recording the hologram, and secondly imaging the resultant diffracted light (reconstructed object beam) onto the detector array with a second lens element of the said. 4f optical design. The following elements are used for reading a reconstructed hologram: a lens element L2 for relaying an object beam reconstructed by directing a reference beam at a storage location in the HRM, having an effective focal distance f2 and which is located at a distance f2 from the HRM in the optical path of a reconstructed object beam; and a light detector, located at a distance f2 from the lens element L2 in the optical path of the reconstructed object beam directed by lens element L2. In the aforementioned 4f optical design f1=f2, but in other optical configurations it may be advantageous for f2>f1 and thereby improve SNR of the reconstructed holograms by use of an optical relay system. Additionally, when magnification or demangnification is preferred in the optical configuration then f2 is not equal to f1.
Typically, an SLM operates in an amplitude mode, whereby the date page appears as a two-dimensional array of dark and bright pixels. When the Fourier transform (FT) of such an amplitude-modulated data page is obtained using a lens element L1, a strong high intensity direct current (DC) spike, that corresponds to the 0th order diffraction, appears at the center of the Fourier transform at the Fourier plane (focal plane) due to the constructive interference of the light from the SLM pixels in the bright state. When large numbers of holograms are multiplexed in the medium, co-locationally or substantially overlapped, this intense DC peak will quickly saturate the dynamic range of a recording material (i.e. deplete the available photopolymerizable component), preventing the efficient use of the medium, and, additionally, will result in significant non-uniformity in the modulation depth of the recording for spatial frequencies of the interference pattern of the holographic recording corresponding to the DC and “alternative current” (AC) components of the FT, and thereby cause poor fidelity in the reconstructed data pages. The intense DC peak at the center and the typically large amplitude distribution in the AC components of the FT spectrum can also result in nonlinear grating formation, increasing the noise level in the reconstructed data page.